Various superconductor devices, such as super-conducting quantum interference devices (SQUIDs), include a heterojunction formed between a superconducting layer and a second layer which is an insulator for tunneling devices, or a metal or semiconductor for proximity effect devices.
More specifically, the Josephson junction comprises a sandwich structure having a barrier layer formed between two superconductors, i.e., a superconductor-insulator-superconductor (SIS) junction, a superconductor-normal metal-superconductor (SNS) junction, or a superconductor-semiconductor-superconductor (SuSmSu) junction. Alternatively, the Josephson junction includes a constriction forming a superconducting microbridge. In either case, a weak link is provided between two superconductors to allow electrons to tunnel or otherwise conduct between the two superconductors resulting in the ac Josephson effect, whereby an oscillating current is induced by the application of a dc voltage across the junction.
Conventionally, such superconducting junctions are formed with low temperature superconductors. Examples of such junctions include NbN-MgO-NbN and Nb-AlO.sub.x -Nb SIS junctions, which have been used as mixer elements in submillimeter wave heterodyne receivers.
The fabrication of similar SIS junctions utilizing high temperature superconductors has proved challenging. To form an epitaxial heterojunction, the barrier material must have a crystal structure and lattice constant closely matching those of the superconducting layers. High quality devices also require a fully-developed energy gap within one coherence length of the superconductor-insulator interfaces. Since high temperature superconductors have coherence lengths of only a few angstroms, interfaces which are of a high quality on an atomic scale are essential, thus placing stringent requirements on the fabrication of SIS junctions utilizing high temperature superconducting materials.
Furthermore, high temperature superconducting materials have various undesirable properties not found in low temperature superconductors, so that low temperature superconductor device fabrication techniques cannot be successfully applied to the fabrication of high temperature superconductor devices. For example, high temperature superconductors react with most commonly used metals and semiconductors, thus preventing the formation of an epitaxial junction.
Previously, attempts have been made to form SIS epitaxial heterojunctions using high temperature superconductors and insulating oxides having perovskite or distorted perovskite crystal structures.
Table I provides crystal lattice structure parameters for some high temperature superconductor materials.
TABLE I ______________________________________ Superconductor (Hole-Type) a (.ANG.) b (.ANG.) c (.ANG.) Structure ______________________________________ YBa.sub.2 Cu.sub.3 O.sub.7 3.82 3.89 11.68 orthorhombic Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+x 3.83 3.83 30.8 orthorhombic Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.8+x 3.855 -- 29.35 body-centered tetragonal ______________________________________
Table II provides crystal lattice parameters for possible insulator barrier materials. As can be seen from Tables I and II, the lattice parameters of the insulators adequately match the lattice parameters of the high temperature superconductor materials.
TABLE II ______________________________________ Barrier (Insulator) a (.ANG.) b (.ANG.) c (.ANG.) Structure ______________________________________ SrTiO.sub.3 3.905 -- -- cubic (perovskite) LaAlO.sub.3 3.78 -- -- monoclinic (pseudo (distorted cubic) perovskite) KTaO.sub.3 3.99 -- -- cubic (perovskite) PrGaO.sub.3 3.863 -- -- orthorhombic (pseudo (distorted cubic) perovskite) ______________________________________
However, despite an adequate lattice match, efforts to fabricate heterojunctions between these materials have been unsuccessful, most likely due to the short coherence length in the high temperature superconductors.
Efforts to fabricate epitaxial SuSmSu junctions have also been unsatisfactory. High temperature superconductors react with common semiconductors, such as Si, thus preventing the formation of a heterojunction. Therefore, efforts to fabricate SuSmSu junctions have focused on oxide semiconductors, which are less likely to be reactive, as the barrier material. In particular, oxide semiconductors such as PrBa.sub.2 Cu.sub.3 O.sub.7 and Nb-doped SrTiO.sub.3 have been investigated. These oxide semiconductors have a proper lattice match with the high temperature superconductor YBa.sub.2 Cu.sub.3 O.sub.7. However, heterojunctions formed from these oxide semiconductors exhibit nonideal current-voltage (I-V) characteristics. The unsatisfactory results are likely caused either by defects in the barrier layer, such as pinholes or other structural inhomogeneities, or by poor interfacial quality.
SNS junctions have been investigated as an alternative to the junctions discussed above. In an SNS junction, superconductivity is induced in a normal metal barrier layer via the "proximity effect," wherein a weak conductive link, analogous to a tunneling link in an SIS junction, is formed across the barrier. Whereas tunnel barrier thicknesses are typically limited to .about.20-30.ANG. for devices with practical current densities, the normal metal bridge length is determined by the normal metal coherence length, which can be significantly longer (.about.100-1000.ANG.). A device using a metallic barrier can therefore have larger dimensions. Fabrication of SNS junctions is thus easier and the requirements for material quality are not as stringent.
However, the cuprate superconductors react with most normal metals to oxidize the metals, thus destroying superconductivity in the interfacial region.
Thus, the fabrication of a high quality SNS junction using only high temperature superconductors requires using a barrier metal which does not react with the superconductor. Further, the barrier metal must be lattice matched to the superconductor at temperatures ranging from the growth temperature of the high temperature superconductor (&gt;700.degree. C.) down to device operating temperatures (&lt;-200.degree. C.). A metal which meets these requirements has not heretofore been identified.
Thus, efforts to form epitaxial heterojunction devices using high temperature superconductors have been largely unsatisfactory. Conventional insulators have not yielded effective SIS junctions because of problems resulting from the short coherence length of the high temperature superconductors, as well as interfacial reactions occurring at the elevated temperature required for growth of the counterelectrode. Conventional semiconductors have not yielded effective SuSmSu junctions with high temperature superconductors because of structural inhomogeneities occurring in the barrier layer or because of poor interfacial quality between the barrier and the superconductor. Finally, conventional metals have not yielded effective SNS junctions with high temperature superconductors because most conventional metals react with high temperature superconductors.
Accordingly, it can be appreciated that there is a need to provide new combinations of materials to achieve epitaxial heterojunctions using high temperature superconductors.